1. Field of the Invention
The present invention relates to an optical pickup device corresponding to an optical disc of at least three formats. The present invention particularly relates to an optical pickup device having a 2-wavelength laser element and a 1-wavelength laser element, and a collimating means having different refractive powers of two systems and an objective lens having two different focal lengths, and a recorder and/or reproducer using the same.
2. Description of Related Art
There is, heretofore, an optical pickup device for recording and/or reproducing an information signal to an optical disc having different formats, such as a CD (Compact Disc) and a DVD (Digital Versatile Disc). This optical pickup device has a laser element for DVD, a laser element for CD, a collimator lens, an objective lens and a light receiving element. The laser element for DVD is formed with a laser diode for emitting a laser beam having a wavelength of 660 nm and a coupling lens in one package, corresponding to the formats of the respective optical discs. The laser element for CD is formed with a laser diode for emitting a laser beam having a wavelength of 780 nm and a coupling lens in one package. The collimator lens parallelizes the laser beams emitted from the respective laser elements. The objective lens converges the respective laser beams to the signal recording surface of each optical disc. The light receiving element receives the reflected beam from each optical disc.
This pickup optical system comprises a laser element for emitting laser beams of two wavelengths and a coupling lens. In this manner, forward path magnifications of the CD light and the DVD light are optimized independently, corresponding to the two disc formats. Recently, in the optical disc, a laser beam in which a track pitch and a bit interval are narrowed and which has a wavelength of 405 nm band, is used, in addition to a CD and a DVD. In this manner, a BD (Blu-ray Disc) in which high recording density is achieved is provided. When an information recording and/or reproducing including such BD is considered, it is desired to set an optical system which can realize separate optical magnifications corresponding to three disc formats.
In the pickup optical system of a disc recorder and/or reproducer corresponding to these three optical discs, a light emitting means for emitting laser beams of three wavelength bands corresponding to the recording formats of the respective discs is required. Furthermore, a lens system having a magnification for converging the optical spot on the signal recording surface of each optical disc in an optical coupling efficiency sufficient to record and/or reproduce the optical spot is required.
Here, it is assumed that the laser beam emitted from the light emitting means for emitting a laser beam having three wavelength bands is incident on the optical disc by using a single optical path common for three wavelengths like a conventional optical pickup system. Then, there arises a difference in the optical magnifications of the lens system needed in response to the formats of the respective optical discs. Therefore, when the optical system is composed by using a lens having a numerical aperture suitable for the format of one optical disc, even if the laser beams corresponding to the other two optical discs pass this optical system, optimum incident angle and output angle are not obtained. As a result, the quantity of light on the signal recording surface of the disc becomes insufficient.
FIG. 1 shows one or a plurality of laser diodes 141, a collimator lens group 142 common for three waves, and an objective lens common for the three waves. The laser diode 141 emits laser beams having wavelengths of 405 nm, 660 nm and 780 nm corresponding to the BD, the DVD and the CD. The collimator lens group 142 parallelizes the laser beams of these wavelengths. The objective lens 143 converges the laser beams of the respective wavelengths to the signal recording surfaces of the corresponding optical discs. The objective lens 143 includes a lens 144, and a hologram element 145 for correcting a spherical aberration generated according to the numerical aperture and the difference of the thickness of cover layers of the optical discs in response to the laser beams of the respective wavelengths. Also, this collimator lens group 142 and the objective lens 143 have magnifications (for example, ten folds) capable of forming an optical spot of optical intensity sufficient to record and reproduce the laser beam having a wavelength of 405 nm on the signal recording surface of the BD, and it is assumed that they are fixed to the arrangement. In such an optical system, when recording of an information signal is performed with the laser beam having a wavelength of 405 nm, since the aperture is limited to a numerical aperture of 0.85 of the BD by a diaphragm (not shown) provided on the objective lens 143 or an actuator. From this, an effective angle of the laser beam incident on the collimator lens group 142 becomes 9.8°, and the luminous flux of the range is converged to the signal recording surface of the BD.
When an information signal is recorded with a laser beam having a wavelength of 660 nm for DVD by using this optical system, the aperture is limited to the numerical aperture of 0.65 of the DVD by the diaphragm (not shown) provided on the objective lens 143 or the actuator. From this, an effective angle of the laser beam incident on the collimator lens group 142 becomes 7.5°, and the luminous flux of the range is converged on the signal recording surface of the DVD. When the information signal is recorded with the laser beam having a wavelength of 780 nm for CD by using this optical system, the aperture is limited to a numerical aperture of 0.52 of the CD by a diaphragm (not shown) provided on the objective lens 143 or the actuator. From this, an effective angle of the laser beam incident on the collimator lens group 142 becomes 6.1°, and the luminous flux of the range is converged to the signal recording surface of the CD.
Here, when an optical system having a magnification of 10 folds is assumed, an effective angle of the laser beam for assuring a quantity of light necessary to record a signal of the BD is, though there is little actual example so far, 8 to 10° based on the present inventor's experiences. Then, no problem occurs when the laser beam is converged to the BD at the effective angle of the 9.8°. On the other hand, since there are many examples in which the effective angle of the laser beam for assuring the quantity of light necessary to record the signal of the DVD is generally designed to be about 11.5° to 14.5°, the quantity of light is insufficient with the laser beam incident at the effective angle of 7.5°. To detect the laser beam without problem, the effective angle of about 1.7 folds is required. There are many examples in which the effective angle of the laser beam for assuring the quantity of light necessary to record the signal of the CD is generally designed to be about 14.0° to 15.5°. Therefore, the quantity of light becomes insufficient with the laser beam incident at the effective angle of 6.1°, and to detect the laser beam without problem, the effective angle of about 2.5 folds is required.
When the difference of the effective angle is converted in terms of necessary laser emitting power, the power of about 2.3 folds is required in the case of the DVD, and the power of about 3.7 folds is required in the case of the CD. Generally, if the recording speed differs four times, the recording power necessary on the signal recording surface requires twice the recording power (2 times faster and 8 times faster, 4 times faster and 16 times faster, or the like). Therefore, in the case of the above-mentioned fixed magnification, even if the laser diode having the same optical output is used, the recording speed can realize only about ¼ in the case of the DVD and only about 1/16 in the case of the CD.
To deal with such problems, an optical system having three optical paths including three light emitting sources and three lens systems to realize the optimum magnifications corresponding to the respective laser beams are composed. Accordingly, when one lens system for realizing the optical magnification suitable for the three disc formats is composed, the number of components for constituting the optical system is increased, and its construction is complicated. Therefore, this optical system cannot respond to the requests of shortening the optical path of the pickup optical system and a size reduction of the pickup device, etc.
[Non-Patent Document 1] Nikkei Electronics, pages 24-25, Jun. 7, 2004.